1. Field of the Invention
This invention relates to smoke detectors of the ionization type and, more particularly, to test apparatus for simulating the presence of a predetermined level of airborne products of combustion within a measuring chamber.
2. Description of the Prior Art
A smoke detector of the ionization type includes an alpha radiation source, such as a small quantity of Americium 241, in a measuring chamber having positive and negative electrodes. The measuring chamber is substantially freely accessible to the atmosphere, including airborne products of combustion. The alpha radiation in the measuring chamber ionizes the air between the electrodes, the result being the flow of a small electrical current when voltage is applied across the electrodes. When airborne products of combustion (smoke) enter the measuring chamber, they reduce the mobility of the ions and thereby increase the resistance of the measuring chamber to the flow of current. The resulting change in the electrical characteristics of the circuit containing the measuring chamber is sensed and used to trigger an alarm when the electrical change reaches a selected level representing a corresponding level of smoke or aerosols within the measuring chamber. The electrical characteristic normally sensed is the change in the voltage across the measuring chamber, the voltage change occurring as a result of the increased chamber resistance due to the presence of visible or invisible products of combustion in the measuring chamber. The sensing or alarm apparatus senses this change in voltage and triggers the alarm when the voltage change reaches the selected level.
It is essential that the smoke detector be highly sensitive and reliable in operation. It is therefore desirable that it be periodically tested to make certain that all of its operative components including the measuring chamber and the alarm apparatus are operating properly. In the past, a common way to test an ionization smoke detector has been to intentionally introduce smoke into the measuring chamber, as by blowing cigarette smoke at the detector, and to assume that everything is working properly in the event that this produces an alarm signal. This approach may not be altogether satisfactory in that there is no way to determine precisely how much smoke actually enters the chamber. For example, for adequate early warning of fires without undue false alarming in response to normal cooking fumes and the like, it is desirable that the alarm be sounded when the smoke level within the measuring chamber is in the range of 2 percent (2 parts per 100). If smoke is blown at the detector, the person testing the system does not know if the alarm has sounded in response to 2 percent smoke or 10 percent or more smoke in the measuring chamber. In other words, an ionization smoke detector may not be operating properly and still pass the "smoke" test. Another test approach has been to provide a test button which, when depressed, introduces into the alarm circuitry an electrical simulation of the measuring chamber characteristics when a predetermined level of combustion product or smoke is present within the chamber. For example, depression of the button in such a system may shunt the measuring chamber with a resistor having a resistance equal to the chamber resistance when the predetermined level of smoke is present within the chamber. It will be readily appreciated by those skilled in the art that this approach adequately tests the performance of the alarm system, but not the operation of the measuring chamber. It is extremely desirable that test means be provided for testing the entire system and all operative components including the measuring chamber and the alarm apparatus.